Is this possible at all?
Issues i see is, over heating, and sliding out they keyboard, unless you use it as a tablet at the time its waterproofed. Let me know, it would be cool..
You could use a zip-lock bag for briefly, maybe, but I suspect heat and condensation would be problems.
For a longer-term setup, you would have to deal with heat, perhaps via heat pipes and a large external heat sink (like the ones used for passive cooling of mid-range graphics cards).
Are any of the Panasonic Toughbooks waterproof? If so, you could see how they solved the problem.